This invention relates generally to composite articles, and more particularly to porous membranes having hydrophilic properties.
Fluoropolymers have excellent chemical and heat resistance properties and in general, are hydrophobic. Expanded porous polytetrafluoroethylene (ePTFE) polymer membranes can be used as filter media for liquid filtration. However, because of the hydrophobicity of fluoropolymers, aqueous dispersions cannot readily be filtered through filters made from these fluoropolymers. Such ePTFE filters can be pre-wetted with organic solvents and then flushed with water to overcome the lack of affinity between the hydrophobic material and the polar aqueous dispersion. However, pre-wetting may be expensive and may lead to “gas-lock” or “dewetting.”
Accordingly, there have been various attempts to make fluoropolymer membranes more hydrophilic and therefore more receptive to wetting with water while maintaining pre-determined properties. Some known methods for making fluoropolymer membranes more hydrophilic include coating a surface of the membrane and the interior of pores of the membrane, with a hydrophilic coating, such as a fluorinated surfactant. However, because the fluoro-surfactant is bound to the membrane only by chemical affinity, the fluoro-surfactant may be inadvertently washed out by the aqueous medium, and as such, the fluoropolymer membrane may therefore lose its water-wettability. Other known methods for making fluoropolymer membranes more hydrophilic include cross-linking a fluoro-surfactant using a radiation beam such as Gamma ray, electron beam, or non-equilibrium plasma. Such cross-linking may not diffuse out the fluoropolymer membrane by the aqueous flow, however the radiation may weaken the mechanical strength of the fluoropolymer membrane. Moreover, the fluoro-surfactant may be adversely effected by the radiation, for example, through deterioration of the fluoro-surfactant and/or alteration of the fluoro-surfactants chemical properties.
Moreover, hydrophilic coatings are sometimes applied to the membrane before the membrane is laminated with a base material or substrate, such as a felt or a woven fabric. However, heat from some lamination processes that bond the membrane to the base material may damage the hydrophilic coating. Furthermore, some membranes may not be strong enough to receive the hydrophilic coating. For example, a weight of fluid filtration membranes may cause the membrane to be damaged by application of the hydrophilic coating, possibly reducing the effectiveness and/or the lifespan of the membrane.